Sorry if this is a repeated question but I bought my first propagator today and it came with some seeds. Unfortunately the seeds (tomato and pepper) instructions say not to sow until April/may time. So I was stuck with just the lettuce. Is it ok to sow them now? Or should guide months always be followed?

Well, we have one member here Odsox who grows tomatoes all year round in his house. I don't do it because I have limited window sill space to do it and my house is not always warm enough as we live in cold squalor. You can sow lettuce now, is your propagator an electric one? lettuce can be grown on a sunny windowsill now, you don't need a propagator for it as there is enough warmth to grow. I only use my propagator from January through to April. I start off things like peppers, aubergines which need a long growing season and tomatoes I want to get going for May use. You could always sow a few tomatoes and pepper seeds if you wanted to and try and keep them going in your house over the winter, you don't need to sow all the seeds

As DnD said, you don't need to use a propagator during the warmer months, it's just a very useful aid to germinate seeds in the first few cold months of the year.
Lettuce would be OK to sow now and if you're feeling adventurous and you have a south facing window in a warm room, then by all means sow a tomato seed. If it's a cordon variety then if you restrict it to 2 trusses it won't take over your room and will be just like any other pot plant.
If it dies then you have only wasted one free seed, so worth a try.

It all depends... I think my basil seeds took about two months to germinate - and that was partly in a propagator, and at the right time of year! My house is so cold, I actually had the heated propagator on at times throughout the so-called summer. And if I did tomatoes from scratch, I would sow them earlier than April, to give them a fighting chance. As I say, depends on your house, how warm you keep the rooms, and how much sun you get in your windows.

It's not an electric propagator, just a plastic thing. It cost £10 in a kit from B&Q with seeds, pots and compost (yes I am that much of a newbie). I've just put them in the front porch (which is an oven!). I have no idea what way any of my windows face..I'll have to buy a compass

The sun comes up in the east and sets in the west. A roasting porch is not a good idea, a sunny windowsill is better than somewhere that gets boiling hot. You need to avoid leggy seedlings, so not letting them get too hot is good.